Mixed-lineage leukemia (MLL) is a proto-oncogene that was originally discovered at the site of chromosomal translocations in human leukemias. Due to chromosomal translocations, MLL is fused with more than 40 different partner proteins to yield a diverse collection of chimeric fusion proteins. The MLL protein is a histone methyltransferase that covalently modifies chromatin and is mutated in certain subsets of acute leukemia. Many of the fusion partners constitutively activate novel transcriptional effector properties of MLL that often correlate with its oncogenic potential in animal models of acute leukemia. MLL normally associates with a group of highly conserved cofactors to form a macromolecular complex that includes menin, a product of the MEN1 tumor suppressor gene. The MEN1 gene is mutated in heritable and sporadic endocrine tumors. Overexpression of menin leads to inhibition of Ras-transfonmed cells. Menin interacts with the transcription factors JunD and NFκB and represses their activation of gene transcription. Studies on these interacting proteins suggest that menin exerts its effects predominantly through inhibitory effects on transcription. However, an alternative possibility is that menin mediates its effects through transcriptional activation of target genes. Additionally, menin interacts with RPA2, a component of a single-stranded DNA-binding protein involved in DNA repair and replication. Menin also interacts with FANCD2, a nuclear protein that plays a critical role in maintaining genome stability with breast cancer 1 gene (Brca1) product.
The exact mechanisms by which menin, which does not have significant homology with other proteins, functions as a tumor suppressor are unknown. Menin plays a role in regulating cellular proliferation because Men1 knockout mice show increased proliferation in neuroendocrine tissues, down-modulation of menin in epithelial cells increases proliferation, and Men1 knockout fibroblasts proliferate more rapidly than wild-type cells as assayed by tritiated thymidine incorporation. MEN1 cells also have increased sensitivity to DNA-damaging agents. Menin interacts with promoters of HOX genes.
Certain oncogenic MLL fusion proteins stably associate with menin through a high-affinity interaction that is required for the initiation of MLL-mediated leukemogenesis. Menin is essential for maintenance of MLL-associated but not other oncogene induced myeloid transformation. Acute genetic ablation of menin reverses Hox gene expression mediated by MLL-menin promoter-associated complexes, and specifically eliminates the differentiation arrest and oncogenic properties of MLL-transformed leukemic blasts.
MLL fusion proteins, a consequence of acquired genetic aberrations, transform hematopoietic cells through two alternate mechanisms, by either constitutive transcriptional effector activity or inducing forced MLL dimerization and oligomerization. Both mechanisms result in the inappropriate expression of a subset of HOX genes, particularly HOXA9, whose consistent expression is a characteristic feature of human MLL leukemias.
Therefore, therapeutic compositions to inhibit interaction of MLL fusion proteins with menin or other MLL binding partners is desired.